The US ambassador to Nato, Nick Burns, has said there is no sense of a crisis in relations with Russia over the expansion of the alliance to seven more countries from central and eastern Europe.

Putin: Russia's nuclear arsenal as deadly as ever.

Russia's Defence Minister Sergei Ivanov earlier warned that Moscow could reassess its military planning in view of what he called Nato aggressive strategy.

Mr Ivanov said he may order a build-up of the country's nuclear defences after Nato announced it would deploy aircraft to defend the airspace of the three ex-Soviet Baltic republics of Estonia, Latvia and Lithuania as soon as they join the bloc.

Over the last few weeks, Moscow has sounded increasingly nervous about Nato's eastward expansion.

One leading MP said most Russians saw the move as a threat to their country, while extreme nationalist Vladimir Zhirinovsky threatened Latvia with utter destruction.

'No threat'

But Mr Burns played down Russia's concerns.

"We've had a year and a half to discuss this with the Russian Federation. I sense no major problem. There is certainly no sense of crisis over this, " Mr Burns said.

He said Nato had no intention of deploying substantial forces in any new member country.

Collective air defence was part of the bloc's role since its foundation, he said, and Russia had been informed about the decision to patrol the airspace of the small Baltic states which have no warplanes of their own.

The decision did not represent in any way a threat to Russia, Mr Burns said.

Deep impact

The Russian Foreign Minister, Sergei Lavrov, is likely to repeat Moscow's concerns on 2 April - when the flags of the seven new member states will be raised outside Nato headquarters in Brussels.

In 1999, Nato admitted three of its former Cold War enemies - Poland, Hungary and the Czech Republic - but this will be the biggest expansion in the alliance's 55-year history.

Nick Burns said the newcomers would have a deep impact, as their accession "shifts Nato's centre of gravity eastward, it extends our reach and it strengthens the alliance".

"When these new countries come in, 40% of our membership will be formerly communist countries and they are going to strengthen us militarily," he said.

New candidates

Washington took a less ambitious approach to Nato expansion before the 11 September 2001 attacks.

But countries like Romania and Bulgaria were quick to open their bases and airspace to US troops on their way to Afghanistan and they soon contributed troops of their own.

Both countries hope to house new US bases as a result of a reassessment of American military deployments all over the world.

But Mr Burns said reports in US media were highly speculative and no decision has been made.

He also made it clear that this wave of expansion would not be the last.

Three Balkans countries - Albania, Croatia and Macedonia - are currently preparing for membership and were invited to the accession ceremony in Washington as observers.

Mr Burns indicated that no other countries were likely to be invited soon.